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Chemical Heterogeneities in the Mantle: Progress Towards a General Quantitative Description
[post]

Massimiliano Tirone

2018
unpublished

<p><strong>Abstract.</strong> Chemical equilibration between two different assemblages (peridotite-type and gabbro/eclogite-type) of variable initial size assuming few different initial compositions has been determined using certain mass and reactions constraints and thermodynamic principles.</p> <p>The pattern that emerges suggests that mass transfer between the two sub-systems defines two petrological assemblages that separately are maintained in local
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... in local thermodynamic equilibrium. In addition, when two assemblages previously equilibrated together in a certain mass ratio are rearranged assuming a different initial ratio, no mass transfer occurs and the two sub-systems remain unmodified.</p> <p>By modeling the chemical equilibration results of several systems it is possible to provide a quantitative framework to determine the chemical and petrological evolution of two assemblages from an initial state, in which the two are separately in chemical equilibrium, to a state of equilibration of the whole system (sum of the two sub-systems). Assuming that the local Gibbs energy variation follows a simple diffusion couple model, a complete petrological description of the two systems can be determined over time and space. Since there are no data to constrain the kinetic of the processes involved, the temporal and spatial scale is arbitrary. Nevertheless a 1-D static model shows how chemical equilibration is controlled by the size of the two sub-systems. As the initial size of the first assemblage (peridotite-like) increases, the differences between the initial and the final equilibrated stage becomes smaller, while on the opposite side the difference increases.</p> <p>A simplified 2-D dynamic model in which either one of the two sub-systems is allowed to move with a prescribed velocity, shows that after an initial transient state, the moving sub-system tends to preserve its original composition defined at the entry side. The other sub-system instead evolves towards a large compositional difference from the starting assemblage. The results appear to be the same varying the initial proportion of the two assemblages, which simplify somehow the development of potential tools for predicting the chemical equilibration process from real data and geodynamic applications.</p> <p>Four animations and data sets of three 1-D and two 2-D numerical models are available following the instructions in the supplementary material.</p>

doi:10.5194/se-2018-67
fatcat:kacndxsgczd3dfmapkkcm7xhzy